The simplest optical data system stores and recalls data by means of either absorption or reflection of a light beam interacting with atoms or molecules comprising a data storage material. A series of data bits can be stored in and read from the storage material by directing a beam of light to spatial storage cells on the material. A "spatial storage cell" is a spatially distinct region of the storage material capable of storing at least one bit of data. An example of a spatial storage cell is a region having a defined area on the surface of the material and extending depthwise, within the length and width boundaries of the cell, into the thickness dimension of the material. A typical unit of data storage material has many millions, if not billions, of storage cells. Each cell can be individually and separately "addressed" for data storage or retrieval by using a laser beam.
The ultimate optical memory device would be one in which a bit of data could be stored in virtually every atom or molecule of the storage material. Such an optical memory would have a storage capacity of about 10.sup.22 bits per cm.sup.3. However, as discussed further below, this ultimate goal is unattainable by current technology.
One promising class of optical memories consists of so-called "frequency-selective optical data storage" (FSDS) memories. A volatile FSDS memory is disclosed in Szabo, U.S. Pat. No. 3,896,420 (Jul. 22, 1975). A relatively non-volatile FSDS memory is disclosed in Castro et al., U.S. Pat. No. 4,101,976 (Jul. 18, 1978).
In general, the spatial dimensions of individual storage cells in optical memories cannot be smaller than .zeta., the wavelength of light employed to add data to and read data from the memory. Since the wavelengths of lasers typically used for writing data into and reading data from optical memories are about 10.sup.-4 cm, the maximal usable number of spatial storage cells in the memory is about 10.sup.12 per cm.sup.3.
FSDS memories are advantageous in that they allow atoms or molecules within a spatial storage cell to be addressed both spatially and spectrally. Since such a memory has the three usual spatial dimensions plus one frequency "dimension," it is often referred to as a "four-dimensional" memory.
FSDS methods utilize data-storage materials in which the storage cells exhibit an "inhomogeneously broadened" absorption profile, as shown in FIG. 1. An inhomogeneously broadened spectral profile having width .DELTA..nu..sub.i in FIG. 1 is a composite of a number of homogeneous spectral lines (e.g., .DELTA..nu..sub.h.sbsb.A, .DELTA..nu..sub.h.sbsb.B, and .DELTA..nu..sub.h.sbsb.C in FIG. 1) each exhibiting a slightly different resonant frequency. Although such data-storage materials typically comprise a substantially uniform assemblage of constituent atoms or molecules, the inhomogeneously broadened spectral profiles of these materials are wider, often much wider, than the absorption profiles of the constituent atoms or molecules. This is because the microenvironment surrounding each individual atom or molecule in the material is different for each atom or molecule which, in turn, imparts a variation in the absorption profile of each atom or molecule. Such microenvironments include small variations in the local electric field surrounding each atom or molecule relative to neighboring atoms or molecules and, in the case of crystalline materials, various imperfections in the crystalline matrix of the material. Thus, each constituent atom or molecule of a storage cell displays a spectrally narrow (i.e, "homogeneous") resonance each having a width of .DELTA..nu..sub.h. The resonances of the individual atoms or molecules in the cell are collectively spread over the inhomogeneously broadened frequency range (having a width of .DELTA..nu..sub.i). Of course, since each storage cell consists of billions of atoms or molecules, it is virtually certain that any given .DELTA..nu..sub.h will be exhibited by more than one atom or molecule in the cell.
In FSDS, if a spatial storage cell of a material having an inhomogeneously broadened spectral profile (i.e., of a material termed an "inhomogeneously broadened absorber" or "IBA") is illuminated by a source of electromagnetic radiation having a specific incident frequency, the entire cell does not undergo a photo-induced change in optical properties. Rather, only those particular atoms or molecules in the cell having a .DELTA..nu..sub.h value at a resonant frequency corresponding to the particular incident frequency undergo such a photo-induced change. This results in formation of a "notch" or "hole" in the inhomogeneously broadened spectrum at the particular resonant frequency (.nu..sub.incid in FIG. 1). Such a frequency-selective saturation phenomenon is termed "hole burning" in the art. Since a number of holes can be burned into the inhomogeneously broadened spectral profile of each storage cell, it follows that, in FSDS memories, one can "write" more than one data bit in each storage cell.
The smallest currently usable spatial storage cell has surface dimensions of about 10.sup.-4 cm on a side. (Individual atoms are about 10.sup.-8 cm across.) Assuming that such a cell has a depth of 10.sup.-4 cm, about 10.sup.12 atoms would be present in the cell. In some storage materials, up to .DELTA..nu..sub.i /.DELTA..nu..sub.h .apprxeq.10.sup.7 frequency subdivisions are possible in each spatial storage cell. Thus, by addressing atoms in the cell both spatially and spectrally, it is possible to individually address groups of atoms containing only about 10.sup.5 atoms each (wherein each group exhibits a different .DELTA..nu..sub.h). To address the same number of atoms with spatial addressing alone, the storage cell would have to be less than 10.sup.-6 cm across, which is too small to be addressable by current technology. With combined spatial and spectral addressing, it is possible to attain data storage densities of 10.sup.18 bits per cm.sup.3 of memory volume, which is 10.sup.7 times denser than what is obtainable with current magnetic memories that offer a surface bit density of about 10.sup.15 bits/cm.sup.2.
Two general types of FSDS memories are known in the art, both of which can (at least in principle) achieve the same ultimate data storage density. The first type is termed "directly addressed" FSDS (also known in the art as "frequency-domain" FSDS) and the second type is termed "temporally addressed" FSDS (also known in the art as "time-domain" FSDS), both of which are briefly described below.
Both FSDS approaches are limited by certain fundamental relationships. For example, in either approach, a monochromatic light source such as a laser is used for data writing and reading. Digital data to be stored ("written") in the memory can be in the form of digital pulses ("ones" and "zeros") superimposed on the substantially monochromatic frequency of the laser. Such pulses are typically created by passing the laser beam through a controllable shutter. If the shutter is open for a time .tau., the resulting pulse passing through the shutter has a duration .DELTA..tau..sub.pulse =.tau.. However, even though the laser beam upstream of the shutter is monochromatic, the pulse passing through the shutter will experience a frequency spread having a width .DELTA..nu..sub.pulse about the laser's monochromatic light frequency .nu..sub.L. Thus, .DELTA..nu..sub.pulse =1/.DELTA..tau..sub.pulse. The longer the pulse, the narrower (i.e., more monochromatic) the frequency spectrum of the pulse. This relationship, termed the Fourier bandwidth theorem, significantly impacts how much data can be written into an optical memory that is spectrally addressed. With memories employing spectral channels of a specific frequency width, a minimum time is required to access a single spectral channel; the narrower the spectral channel, the longer the access time required.
In "directly addressed" FSDS such as disclosed in Burland, U.S. Pat. No. 4,158,890 (Jun. 19, 1979), and in Szabo, U.S. Pat. No. 3,896,420 (Jul. 25, 1975), highly monochromatic lasers are utilized for data storage and retrieval. A "data-write" laser is spatially positioned to address the various spatial storage cells on a storage material and frequency-swept to address individual spectrally distinguishable subgroups of atoms or molecules within each spatial storage cell. (Each such spectrally distinguishable subgroup is termed a "spectral channel"). Within a spectral channel, an "on" bit is represented by a write-induced change in the absorptivity of the active atoms or molecules in the channel.
The ultimate narrowness of the spectral channels is determined by the homogeneous absorption bandwidth .DELTA..nu..sub.h of the corresponding storage material. The homogeneous absorption bandwidth .DELTA..nu..sub.h is an expression of the range of light frequencies (about an atom- or molecule-specific median frequency) within which an atom or molecule in the spectral channel will strongly interact with an incident light beam. If spectral channels are spaced at less than .DELTA..nu..sub.h, writing data to one spectral cell will affect data stored in spectrally adjacent cells. Therefore, maximal storage density is achieved when the spectral channel width is equal to .DELTA..nu..sub.h.
The drawbacks associated with directly-addressed FSDS arise from a need to rapidly tune the read and write lasers to specific frequencies. This is because the data bits are written (and read) sequentially directly into (and from) frequency-domain spectral channels. Also, a spectral channel of width .DELTA..nu..sub.h cannot be individually accessed in a time shorter than about .DELTA..nu..sub.h.sup.-1. Therefore, the data input/output rate of directly addressed FSDS memories is rather slow, limited to about .DELTA..nu..sub.h bits per second. (The maximum speed at which holes can be burned is determined by the relationship .DELTA.t.sub.h.DELTA..nu..sub.h .apprxeq.1, where .DELTA..nu..sub.h is the single-channel access time and .DELTA..nu..sub.h is the spectral width of the channel.) For example, in a storage material having 10.sup.7 spectral channels, the minimal spectral channel width is equal to .DELTA..nu..sub.h =10.sup.3 Hz. With such a channel width, the maximum data transfer rate is only 1 Kilobit/sec. Since a cm.sup.3 of such a material could contain 10.sup.18 bits of information, it would take millions of years to write or read such a memory at 1 Kilobit per second.
Therefore, it is impossible using directly addressed FSDS to simultaneously optimize storage density and memory speed. I.e., if the storage capacity of the absorbing material in the memory is increased by making each spectral channel narrow, one unavoidably increases the time needed to create and retrieve information from that spectral channel.
"Temporally addressed" FSDS is also termed "time-domain" FSDS or "stimulated echo FSDS" in the art. Mossberg, U.S. Pat. No. 4,459,682 (Jul. 10, 1984); Carlson et al., Phys. Rev. A 30:1572 (1984); Bai et al., Opt. Lett. 11:724 (1986); Mossberg, Optics Lett. 7:77-79 (1982). In temporally addressed FSDS, the maximum permissible rate at which storage cells are accessed is also dependent upon the material comprising the data storage medium. During data "writing," each spatial storage cell is exposed to a writing laser beam for a time that can be as long as .DELTA..nu..sub.h.sup.-1. However, unlike directly addressed FSDS, each spectral channel within a spatial storage cell can be simultaneously addressed. I.e., .DELTA..nu..sub.i /.DELTA..nu..sub.h bits (one for each available spectral channel) can be simultaneously encoded and stored in the cell. The storage of all these bits in the cell occurs in parallel during a time interval equal to that required to store a single bit in a cell using directly addressed FSDS.
In temporally addressed FSDS, two types of pulses are involved in a "writing" sequence (the storage of a bit stream in a spatial storage cell). One of said pulses is termed the "reference pulse" (also termed "preparation pulse," "write pulse," or "fixing pulse" in the art). The reference pulse can be temporally short compared to the bandwidth of the bit stream to be stored. The second pulse, termed the "data pulse" (also termed the "object pulse" or "writing pulse" in the art) consists of the actual stream of data bits to be stored in the cell.
The data pulse is produced by passing a laser beam through a high-speed laser shutter so as to digitally encode a train of binary bits onto the laser beam. The frequency content of such a data pulse is complex and unique to the particular data stream encoded therein. Since a temporally encoded laser pulse carries a unique frequency-spectrum signature, storage of such a spectrum in a cell constitutes a storage of all the temporally encoded data. By illuminating a single spatial storage cell using a temporally encoded data pulse, many of the spectral channels in the cell are simultaneously addressed.
The reference pulse is necessary for recording the data in the cell. If the cell is exposed only to the data pulse, then only the power spectrum of the data pulse is recorded, which is an insufficient basis for later recall (reading) of the data from the cell in a temporally faithful manner.
The reference pulse can either precede the data pulse or follow the data pulse. The temporal order of these pulses is a determinant of whether the data in the data pulse can be recalled from the cell as a faithful copy of the data pulse or as a time-reversed copy of the data pulse. See, Mossberg, U.S. Pat. No. 4,459,682; Carlson et al., Phys. Rev. 30:1572-1574 (1984). In either event, the reference and data pulses sequentially enter the cell.
Thus, in temporally addressed FSDS, it is the Fourier spectrum of the bit stream in the data pulse that is recorded in the spectral channels of the addressed cell. Because the write and data pulses constructively and destructively interfere with each other in exciting the active atoms or molecules comprising the cell, the time separations and phase relationships between these laser pulses are, in effect, holographically impressed on the absorbing atoms or molecules. This results in storage of information in the form of modulations in the hyperfine energy sublevels of the absorbing atoms or molecules as a function of absorption frequency. That is, the Fourier spectrum is represented through frequency-dependent changes in the absorption profile of the atoms or molecules comprising the cell. The spectral distribution of the atoms or molecules in the cell, after being exposed to the preparation and object pulses, is proportional to the Fourier transform of the object pulse.
Temporally addressed data in a storage cell are recalled or "read" by stimulating the storage cell to emit a coherent optical signal ("signal pulse") having the same duration and temporal envelope as the stored bit stream. Data reading is usually initiated by a single laser pulse, called the "read" pulse, which can be identical to the reference pulse. In effect, the read pulse causes the previously stored data to be holographically reexpressed. The signal pulse, then, is a result of constructive and destructive interactions of energy discharged from the previously stimulated atoms or molecules of the cell. The signal pulse can also be envisioned as an "echo" of the data pulse, wherein the signal pulse has the same temporal envelope as the data pulse. I.e., the temporal profile of the signal pulse is proportional to the Fourier transform of the spectral distribution of the excited population of atoms or molecules.
The signal pulse is a coherently emitted signal (which makes it highly directional). If the pulses involved in the writing sequence are colinear, the signal pulse will have directional properties essentially identical to the data pulse. In general, the wavevector of the signal pulse is related to the wavevectors of the pulses involved in the write sequence according to: EQU k.sub.s =k.sub.d +k.sub.r -k.sub.w
where k.sub.w, k.sub.d, k.sub.r, and k.sub.s are the wavevectors of the write (reference), data, read, and signal pulses, respectively.
As discussed above, in the directly addressed FSDS method, data input and output rates are limited by the spectral channel width, and by the fact that each spectral channel within each spatial storage cell must be serially addressed. With temporally addressed FSDS, the ability to address all spectral channels of a spatial storage cell in parallel provides a substantial speed advantage of .DELTA..nu..sub.i /.DELTA..nu..sub.h over directly addressed FSDS. However, the total duration of the write sequence (write and data pulses) for each channel must be shorter than the homogeneous dephasing time of the storage material. Data input and output must occur at a rate of .DELTA..nu..sub.i bits per second in order to utilize the full storage potential of the storage material. In cases where .DELTA..nu..sub.i exceeds obtainable rates of data input and output, only .DELTA..nu..sub.m /.DELTA..nu..sub.h bits (where .DELTA..nu..sub.m is the maximum achievable rate of data input and output; i.e., the maximum achievable data modulation rate) can be stored using temporal addressing FSDS. This can also pose a substantial rate limitation because full inhomogeneous absorption bandwidths often exceed currently achievable modulation rates.
Therefore, there is a need for optical memories that allow one to achieve complete spectral utilization of the storage material while employing variable rates of data input and output.